The present invention relates to a dual purpose antenna, that is an antenna which is capable of operating with signals in widely separated parts of the radio spectrum simultaneously, and in particular to a dual purpose antenna which has a low physical profile.
Generally speaking, an antenna is designed to operate in a relatively restricted region of the radio spectrum and is optimised for operation in that region.
Recent work in the field of mobile communications has led to a requirement for radio operation in widely separated regions of the radio spectrum. In mobile cellular radio systems, mobile transceivers communicate with one another via a network of fixed base stations using signals in the UHF part of the spectrum. On the other hand, mobile location systems such as Datatrak (RTM) use signals from static locator beacons transmitted at very low frequencies to enable equipment (known as a mobile location unit or MLU) on a vehicle or other moving object to determine its location for any of a number of purposes, and the location determined by the MLU is reported to a base station via a UHF transmission from the mobile for purposes such as monitoring the position of the mobile.
In addition to monitoring the position of a mobile for reporting back to a base station it has also been proposed to make use of a mobile location unit for other purposes. For example, in the case of a mobile equipped with a cellular radio transceiver, optimum values of various operating parameters of the transceiver depend on its position and a MLU may be used to adapt or condition the operation of the transceiver according to its calculated position. For example the calculated location may also be used to adapt or condition the operation of a mobile's cellular radio transceiver to local characteristics of the cellular radio network, for example what transmitter power and which frequency channels to use. (see our British Patent No 87/11490 "Mobile Transmitter/Receiver").
The wavelengths of radio waves at the frequencies used in applications such as cellular radio and the data transmissions used in systems such as Datatrak on the one hand and low frequency mobile location systems on the other differ by several orders of magnitude making it difficult to design a single antenna which is usable with both.
The present invention provides a dual purpose antenna usable with radio signals in two widely separated regions of the radio spectrum simultaneously and which comprise high frequency and low frequency sections usable with signals in the higher and lower of the two regions respectively, the high and low frequency sections being integrated into an antenna assembly which comprises an antenna arrangement tuned and loaded for operation in the high frequency region and a voltage probe for receiving the E-component of signals in the low frequency region.
The antenna arrangement may include a number of antenna elements, one of which serves also as the voltage probe.
In particular, it may comprise first and second planar conductive antenna elements separated by a dielectric, the first element being a radiating/receiving element for the high frequency signals and the second element serving both as part of a resonant circuit including the first element in its high frequency operation and as the LF voltage probe.
The HF section may have a third, linear radiating element whose axis extends out of the plane of the first and second elements from the centre of the first element, whereby the antenna acts to radiate signals in the high frequency region omnidirectionally, the radiated signals being polarised in the direction of the axis of the third element.
Using a voltage probe to pick up the E-component (electric component) of the low frequency signal frees the antenna from having its dimensions constrained by the wavelength of the low frequency signals.
As will become apparent from the following description, the present invention permits a dual purpose antenna to be produced which is physically compact and of a low profile which is convenient in itself and enables the antenna to be packaged in an enclosure which is resistant to tampering, (e.g. by someone attempting to disable communication from the mobile), while permitting a single-point fixing to the roof of a vehicle or other moving object.
In one particularly convenient form, the antenna elements are disposed as two electively conductive areas of metal foil on a dielectric substrate, with the first element being in the form of a circular disk which is concentric with and spaced from the second element which takes the form of a circular annulus.